Successful fabrication of biaxially textured superconducting wire based on the coated conductor technology requires optimization of the cost/performance of the HTS (high temperature superconductor) conductor. From a superconducting performance standpoint, a long, large area, flexible, single crystal-like wire is required. From a cost and fabrication standpoint, an industrially scalable, low cost process is required. Both of these critical requirements are met by Rolling-assisted-biaxially-textured-substrates (RABiTS).
In order for cost/performance for a conductor based on this technology to be optimized, further work needs to be done in the area of buffer layer technology. It is now clear that while it is fairly straightforward to fabricate long lengths of biaxially textured metals or alloys, it is quite difficult to deposit high quality buffer layers using low cost processes. Requirements of buffer layers include—it should provide an effective chemical barrier for diffusion of deleterious elements from the metal to the superconductor, provide a good structural transition to the superconductor, have a high degree of crystallinity, excellent epitaxy with the biaxially textured metal template, have good mechanical properties, high electrical and thermal conductivity and should be able to be deposited at high rates.
Buffer layers play a key role in high-temperature superconductor (HTS) materials, particularly in second-generation rare-earth-barium-copper-oxide (REBCO), especially yttrium-barium-copper-oxide (YBCO) wire technology. The purpose of the buffer layers is to provide a continuous, smooth and chemically inert surface for the growth of the YBCO film, while transferring the biaxial texture from the substrate to the YBCO. Buffer layers are important for preventing metal diffusion from the substrate into the superconductor, and as oxygen diffusion barriers. Nickel can poison the YBCO layer, destroying the superconductive properties. To transfer the texture from the template to the superconductor while preventing the diffusion of nickel and tungsten metal to the YBCO film, buffer layers are needed. These insulating layers also reduce both alternating current (ac) losses and the thermal expansion mismatch between the crystal lattices of the substrate and the superconductor. Multi-layer architectures have been developed that also provide mechanical stability and good adhesion to the substrate.
It is important that the highly aligned buffer materials are matched in both the lattice constant and thermal expansion to the biaxially textured substrate and the YBCO layer. The buffer layers should be smooth, continuous, crack-free and dense. Even though the buffer layers are much thinner than the YBCO layer, buffer deposition cost is a substantial part of the total conductor cost.
When growing an epitaxial oxide film on a metal surface it is essential to avoid oxide formation before the nucleation of the layer is complete. For example YBCO is typically grown in an atmosphere containing 100 ppm or more of oxygen at 750-800° C. Under such conditions Ni and W will form various native oxides on a NiW surface. Most of such oxide layers do not offer the bi-axial cubic texture required for high critical currents in the HTS layer. However, the ability of certain oxide films to be grown in very low oxygen partial pressures, i.e. below the oxidation of Ni and even W can be utilized.
Although it is possible to grow oxide buffers directly on textured metal surfaces under suitable reducing conditions, oxygen penetrates through the buffer layers such as yttrium oxide, yttria stabilized zirconia, cerium oxide, and lanthanum zirconium oxide (LZO) to the metal oxide interface during the YBCO processing. This is mainly because the oxygen diffusivity of the buffer layer materials at 800° C. is in the range of 10−9 to 10−10 cm2/sec. The diffusion is more rapid in structures that are more prone to the occurrence of defects. Grain boundaries, pinholes and particulates can also provide diffusion paths in these films. In most instances the oxidation of the metal substrate cannot be suppressed completely. Thin homogeneous oxide layers are observed after final processing of the conductor without negative impact on the performance. Uncontrolled growth of substrate oxides, however, can result in multiple failures. Rapid and inhomogeneous oxide growth can penetrate the buffer layers and cause deterioration of the barrier properties of the buffer layer. For example, when excessive NiO is formed the full buffer layer stack can delaminate from the metal substrate due to volume expansion of, for example, Ni relative to NiO. The characteristics of the buffer layer can control the extent of oxide layer formation at the interface between the buffer and the substrate.
Buffer layers can be formed by multiple layers of materials which each serve a particular purpose. A thin seed layer is deposited directly on the substrate first, to subsequently allow the growth of the full buffer and finally the YBCO processing at higher oxygen levels. The function of a seed layer is to protect the substrate from oxidation during the formation of subsequent layers, which can include additional buffer layers and the superconductor layer, and also to provide an epitaxial template for growth of these subsequent layers.
One example of a buffer layer stack of the prior art includes the use of YSZ and CeO2, typically in a configuration of CeO2 (or Y2O3)/YSZ/CeO2. The purpose of the first buffer layer, or seed layer, is to provide a good epitaxial oxide layer on the reactive, biaxially textured Ni substrate without the formation of undesirable NiO. CeO2 is particularly useful in this regard due to its ability to very readily form single orientation cube-on-cube epitaxy on cube textured Ni. Deposition of CeO2 using a range of deposition techniques can be done using a background of forming gas (4% H2-96% Ar) in the presence of small amounts of water vapor. Under such conditions the formation of NiO is thermodynamically unfavorable while the formation of CeO2 is thermodynamically favorable. The water vapor provides the necessary oxygen to form stoichiometric CeO2. Using CeO2 as a buffer layer one can readily obtain a single orientation, sharp cube texture. Ideally, the CeO2 layer would be grown thick such that it also provides a chemical diffusion barrier from Ni, followed by deposition of YBCO. However, when the CeO2 layer is grown greater than 0.2 μm in thickness, it readily forms micro-cracks. Hence a YSZ layer which does provide an excellent chemical barrier to diffusion of Ni and does not crack when grown thick is deposited on a thin initial template of CeO2. However, since there is a significant lattice mismatch between YSZ and YBCO, a second phase with 45°-rotated orientation nucleates at times. In order to avoid the nucleation of this second orientation completely, a thin capping layer of CeO2 or another material is deposited epitaxially on the YSZ layer.
The fabrication of the buffer layer stack can be a time consuming, and cost limiting, factor in the process of fabricating superconducting articles. Accordingly, there is a need for improved methods of forming buffer layer stacks and in particular the seed layer. Efforts are being made to replace the existing three layer RABiTS architecture of all-sputtered CeO2 (or Y2O3)/YSZ/Y2O3 with buffers deposited by industrially scalable solution methods. Metal organic deposition (MOD) processes offer significant cost advantages compared to physical vapor deposition (PVD) processes. About 0.8 μm thick MOD-YBCO films grown on sputtered-CeO2 capped MOD-LZO films processed at 900° C. supported an Ic of 336 A/cm width (Jc of 4.2 MA/cm2) at 77 K and self-field. This performance demonstrated the barrier properties of MOD-LZO and exceeded the typical 300 A/cm performance of all-PVD buffers. The barrier properties of LZO were demonstrated by growing MOD-LZO on sputtered Y2O3 seeds. Sputtered Y2O3 seed layers improve the texture relative to the substrate texture. The challenge is to develop an MOD seed layer with improved texture. MOD-Y2O3 seeds grown directly on textured Ni-3W substrates did not show any improvement. However, recently, the growth of MOD-La3TaO7 (LTO) and MOD-CeO2 seeds with improved texture on textured Ni-3W substrates has been demonstrated. MOD-LTO seeds were not compatible with MOD-LZO and CeO2 and also with sputtered-YSZ barriers. However, MOD-CeO2 seeds were compatible with sputtered YSZ and CeO2 and high Ic YBCO films were demonstrated. It is important to limit the thickness of CeO2 seeds to 40 nm or less to avoid the crack formation. Hence, there is a real need to develop another suitable solution seed with improved texture.